The present invention relates generally to wavelength division multiplexing, and more particularly, to minimizing losses from thermal and pressure effects on wavelength division multiplexing/demultiplexing devices.
The telecommunications industry has grown significantly in recent years due to developments in technology, including the Internet, e-mail, cellular telephones, and fax machines. These technologies have become affordable to the average consumer such that the volume of traffic on telecommunications networks has grown significantly. Furthermore, as the Internet has evolved, more sophisticated applications have increased data volume being communicated across telecommunications networks.
To accommodate the increased data volume, the telecommunications network infrastructure has been evolving to increase the bandwidth of the telecommunications network. Fiber optic networks that carry wavelength division multiplexed optical signals or channels provide for significantly increased data channels for the high volume of traffic. The wavelength division multiplexed optical channels are comprised of narrow band or substantially monochromatic optical signals. The wavelength division multiplexed optical channels carry data packets containing information, including voice and data. Contemporary optical networks can include forty or more substantially monochromatic optical channels on a single fiber and each substantially monochromatic optical channel can carry many thousands of simultaneous telephone conversations or data transmissions, for example.
An important component of the fiber optic networks is a wavelength division multiplexer (WDM). A WDM is utilized to multiplex and demultiplex the wavelength division multiplexed optical signals to and from individual fibers in the fiber optic networks.
A WDM includes optical components that, in the case of demultiplexing, separate polychromatic optical signals into individual substantially monochromatic optical signals, and, in the case of multiplexing, combine substantially monochromatic optical signals into polychromatic signals. The optical components of one type of WDM include lenses for focusing and collimating the optical signals and a diffraction grating for diffracting the optical signals to perform the multiplexing and demultiplexing functions. Optionally, a prism is included. A diffraction grating component can comprise a moldable or castable material, such as epoxy, into which the diffraction grating profile is pressed, an optical reflective coating, such as gold or aluminum, that is coated onto the material, and a substrate into which the material is attached. The grating substrate provides thermal stability to maintain groove spacing of the diffraction grating. A support structure is used to either mount or house the optical components of the WDM.
Both the mechanical and optical components of the WDM are affected by changes in temperature. They expand and contract changing in shape and relative position, and also changing in optical properties. While the WDM inherently has losses, the changes due to temperature and pressure variations increase the inherent losses and affect the ability of the WDM to effectively transmit an optical signal. It is desirable to minimize the increase in losses, and therefore, there is a need for a WDM device that is pressure and/or thermally compensated, such as being abaric and/or athermal.
To overcome the adverse affects of changes in temperature and/or pressure, a WDM has been designed to pressure and/or temperature compensate. The WDM has a structure for holding at least one optical component. A diffraction grating assembly having a substrate is held in relation to the at least one optical component by the structure. A lens assembly having a focal length is held in relation to the at least one optical component. The coefficient of thermal expansion of the lens assembly and structure are approximately equal. The lens assembly is constructed from a material chosen to minimize its variance in focal length over temperature. The grating assembly has an angular dispersion that changes with temperature and the product of the focal length and angular dispersion remains constant over temperature. The WDM further comprises a prism having a change in index of refraction with temperature that is approximately equal to a negative of a coefficient of thermal expansion of the substrate. In the absence of a prism, the grating substrate has a coefficient of thermal expansion approximately equal to a negative of a coefficient of thermal expansion of air.